It is known to use window shades for regulating the entry of light into the interior of motor vehicles. Such window shades customarily have a winding shaft, which is arranged in a concealed manner, and on which an edge of the window shade web is fastened. The window shade web is prestressed by a spring drive in a direction for winding up the window shade web. The window shade web is extended through a slit provided in the window sill, the upper roof edge, or the rear window shelf.
A pull rod, which is guided at an end in at least one guide rail, is provided for maintaining the window shade web taut. If two guide rails are used, the pull rod is guided at both ends, generally with a telescope-like extension or shortening of the pull rod during its retraction and extension, since the windows of motor vehicles are not exactly rectangular.
The displacement of the pull rod along the guide rods against the effect of the spring drive takes place with the aid of linearly-shaped flexible actuating elements. Such flexible actuating elements are designed similar to a Bowden cable and consist of a cylindrical core or a cylindrical core element, which supports a helically extending rib on its exterior. A toothed rack is created in this way which is provided with teeth all around. The exterior diameter of the actuating element is designed such that it can be guided, substantially in a buckle-like manner, in the groove of the guide rail without the actuating element being able to come out of the slit of the guide groove. In arrangements known up to now, there is a flush connection between the respective end of the actuating element and the guide body of the pull rod running in the guide groove. This flush connection can only transfer pressure forces.
A gear motor is provided for extension and retraction of the window shade, whose output shaft supports a gear wheel which meshes with the actuating element. Displacement of the actuating element causes the window shade web to be pulled off the winding shaft to extend the window shade web. In the course of that movement, the window shade web is maintained taut by the force of the spring motor. Retraction of the window shade takes place by changing the direction of rotation, with the electric motor pulling the actuating element back along the guide groove in order to allow the pull rod to move in the direction toward the winding shaft.
With an aim of utilizing the smallest possible space for the winding shaft, the winding shaft typically has a small exterior diameter. Accordingly, a great number of revolutions are required for rolling up the window shade web in its entirety. Even when using a spring drive with a relatively long spring travel, the winding torque at the end of the retraction differs considerably from that at the start. Added to this is that, with the window shade completely rolled up, the effective diameter, and therefore the lever arm, is large. With an acceptable tension in the shade web when it is completely extended, the force pulling the pull rod back becomes comparatively weak toward the end of the retraction movement.
Under unfavorable conditions the pull rod can become stuck before it has been completely retracted. The seizing can be aided by small clamping forces of the guide bodies in the guide rail. Because of the change in width, the guide bodies must be prestressed to assure freedom from rattling, and as a result, are subjected to considerable friction in the guide rail.